This application claims the priority of Japanese Patent Application No. 11-066067, filed on Mar. 12, 1999 in Japan, the entire contents of which are hereby incorporated herein by reference.
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1. Description of Related Art
Thin film fabrication processes such as sputtering and CVD are widely employed in the manufacture of LSI and various other electronic devices and display devices such as liquid crystal displays, etc. Meanwhile, greater and greater advances are being made in device integration and miniaturization in the field of semiconductor devices. Device miniaturization demands new techniques in device manufacture. More specifically, aspects involved comprise the filling of very fine holes with sufficient amounts of film, measures to reduce step differences in device manufacture, and the prevention of lead breakage due to electron migration or the production of heat caused by high current density. In particular, technology by which a barrier film with sufficient thickness is formed with good coverage in the bottom of a fine hole with a high aspect ratio (the ratio of the hole depth to the diameter or width of the hole""s opening) is a technology which holds the key to future semiconductor device manufacture.
A barrier film is used for the purpose of preventing mutual attack and diffusion of underlying material and wiring material (ensuring a barrier characteristic) and also for the purpose of ensuring electrical conductivity and ensuring close adhesion. Laminate films consisting of titanium films and titanium oxide films, tantalum films, tantalum oxide films and laminate films consisting of tantalum films and tantalum oxide films, etc. are used as barrier films.
There is currently interest in the plasma CVD procedure and ionizing sputtering procedure, etc. in which a film is deposited while imposing a bias on a substrate as procedures for forming a barrier film on the inner surface of a fine hole which has a high aspect ratio. FIG. 9 will be used to describe an ionizing sputtering procedure as one example of conventional procedures. FIG. 9 is a front view which schematically shows the structure of an ionizing sputtering apparatus constituting one example of a conventional thin film fabrication apparatus.
The apparatus shown in FIG. 9 comprises a processing chamber 1 whose interior is pumped out by a vacuum pump system 11, a substrate holder 2 which holds a substrate 9 in a set position in the processing chamber 1, a gas delivery system 3 which introduces a set process gas into the processing chamber 1, and a plasma generation means which produces a plasma in the processing chamber 1. The plasma generation means consists principally of a cathode 4 with a target 41 installed such that the surface which is to be sputtered is exposed in the processing chamber 1, and a sputter power supply 5 which causes a plasma to be produced by imposing a set voltage on the target 41 and causing sputtering discharge to be produced.
A source which imposes a high-frequency voltage with a frequency of about 13.56 MHz on the target 41 is used as the sputter power supply 5. When a set process gas is introduced into the processing chamber 1 by the gas delivery system 3 and a high-frequency voltage is imposed on the target 41 by the sputter power supply 5, high-frequency discharge occurs in the process gas, and a plasma is produced. A capacitance for which a matching unit (not shown) is used is present between the sputter power supply 5 and the target 41 surface which is to be sputtered. On imposition of the high-frequency voltage via the capacitance, electrons and ions in the plasma act in charging and discharging of the capacitance, and, because of the difference between the mobility of electrons and that of ions, a self-bias voltage is produced in the target 41. The self-bias voltage is a negative direct-current voltage which is superimposed on the high-frequency voltage. Because of this self-bias voltage, ions are drawn out from the plasma and strike the target 41. As a result, the target 41 is sputtered.
Particles (which are normally in an atomic state, and are referred to below as sputter particles or sputter atoms) which are ejected from the target 41 by sputtering fly through the interior of the processing chamber 1 and reach the surface of the substrate 9. Arrival of a lot of sputter particles results in the growth of a thin film. When sputtering is effected for a set time, a thin film with a set thickness is produced on the surface of the substrate 9.
The sputter power supply 5 also serves as an ionization means which ionizes sputter particles which are ejected from the target 41 through sputtering. When a high-frequency power supply such as noted above is used as the sputter power supply 5, electrons in the plasma collide with the sputter particles, so increasing the efficiency of ionization of the sputter particles. Sometimes, a structure in which a high-frequency voltage is imposed on a high-frequency electrode which is provided partway along the flight path of the sputter particles is employed as an ionization means.
There is further provided a bias system 6 which biases the substrate 9 in order to cause ions in the plasma to strike the substrate 9. What is meant by xe2x80x98biasxe2x80x99 is that a set potential relative to the plasma""s space potential is imparted to the surface of the substrate 9 in order to cause ions in the plasma to strike the substrate 9.
The substrate holder 2 comprises a holder main body 21 made of metal which is kept at ground potential, and a dielectric block 22, etc., which is fixed to the holder main body 21. A bias electrode 23 is provided inside the dielectric block 22. The bias system 6 in a conventional apparatus consists mainly of a bias power supply 61, etc. connected to the bias electrode 23.
When sputtering is effected while using the bias system 6, the action of sputter particles which have been ionized (and are referred to below as ionized sputter particles) has the effect of improving the coverage in holes. This point will now be described with reference to FIG. 10. In FIG. 10, which is a drawing for the purpose of description in relation to the surface potential of the substrate 9 in a conventional method and apparatus, FIG. 10(1) indicates the voltage imposed on the bias electrode 23, and FIG. 10(2) indicates the surface potential of the substrate 9.
First, the substrate 9 is set on the dielectric block 22 constituting part of the substrate holder 2. Therefore, the potential (referred to below as the substrate surface voltage Vs) of the substrate 9 surface which is exposed to the plasma first becomes a floating potential. The floating potential (indicated as Vf in FIG. 10(2)) is a negative potential of around several volts. The strength of the sheath field produced by the floating potential Vf depends on the plasma density. Further, the plasma density depends on the output of the high-frequency power supply which is used as the sputter power supply 5.
On the other hand, the plasma""s space potential (indicated as Vp in FIG. 10(2), is a positive potential of about 0V to several volts. It is thought that the plasma space potential shifts slightly to the positive side because the system attempts to establish a balance as the result of electrons migrating to the surface of the substrate holder 2, etc. A sheath field whose potential gradually falls going toward the substrate 9 is produced between the plasma which is at a space potential Vp such as this and the substrate 9 to which the floating potential Vf is imparted. The orientation of the sheath field is normal to the substrate 9, and ionized sputter particles are accelerated by the sheath field and are incident on the substrate 9 at approximately right angles. As a result, the number of ionized sputter particles which reach as far as the bottom surface of a hole becomes large, and the rate of film deposition on the hole""s bottom surface increases.
However, the situation is not that all the sputter particles ejected from the target 41 are ionized, but neutral sputter particles too arrive in considerable quantity at the substrate 9. These neutral sputter particles are not affected by the sheath field, and they are incident at a variety of angles on the substrate 9. Consequently, these sputter particles cause considerable build-up at the hole opening edge portion, and a protuberance which is called overhang is liable to be formed. If an overhang is formed, the hole""s opening becomes smaller, and its apparent aspect ratio becomes higher. When the aspect ratio becomes high, the amount of the sputter particles which can penetrate into the hole falls, with the consequence that the hole bottom surface coverage deteriorates.
The power supply used for the bias power supply 61 is a high-frequency power supply with a frequency of about 13.56 MHz, for the sputter power supply 5. Therefore, in a state in which a plasma is being produced, when the bias power supply 61 is actuated, a self-bias voltage Vdc is imposed on the substrate 9, as indicated in FIG. 10(2), because of the difference between the mobility of electrons and that of ions. As a result, the state becomes one in which the field produced by the self-bias voltage Vdc is superimposed on the field produced by the floating potential Vf. Since, as noted earlier, the self-bias voltage is the voltage of a negative direct-current component, the sheath field becomes higher still, and the incidence energy of ions increases.
It is noted that, in actual situations, since the voltage (referred to below as the electrode imposed voltage Ve) which is imposed on the bias electrode 23 is a high-frequency voltage, and a field due to this high-frequency voltage exists, the substrate surface potential Vs varies sinusoidally in the manner indicated by the dotted line in FIG. 10(2). Further, the difference between the floating potential Vf and the substrate surface potential Vs constitutes the self-bias voltage Vdc.
However, the frequency of variation of the substrate surface potential Vs is 13.56 MHz, which is considerably higher than the plasma ion oscillation frequency (about 3.3 MHz in the case of an Ar plasma with a density of about 1010 ions/cm3. Therefore, the high-frequency component may be ignored in cases where the behavior of ions is considered a matter of concern. That is, prior to the migration of ions following the high-frequency field, the orientation of the field changes, and so, overall, the high-frequency field does not affect the migration of the ions. Therefore, the principal fields which cause ions to migrate are the field produced by the self-bias voltage Vdc and the field produced by the floating potential Vf. Since the floating potential Vf is also a field which is produced by the difference between the mobility of electrons and that of ions, it is even possible to consider just one type, which is that of the xe2x80x98self-bias voltagexe2x80x99. The situation in this case is that the entire direct-current component of the substrate surface Potential Vs shown in FIG. 10(2) is a self-bias voltage.
As described above, when the energy of incident ions becomes high, the incidence energy of the process gas ions exceeds the sputtering threshold value at the time of sputtering of a deposited film, and there is a possibility of resputtering of the deposited film of an overhang portion. As the result of resputtering of the overhang, reduction of the hole opening is prevented and, since the sputter particles ejected through resputtering fall into the hole, the coverage of the hole""s inner surface (bottom surface and side surface) is improved.
2. Field of the Invention
The present invention relates to a method and apparatus for fabricating a set thin film on a substrate surface by a procedure such as sputtering or plasma-assisted chemical vapor deposition (plasma CVD).
However, with conventional methods and apparatus, problems may arise. Specifically, as the result of the sheath field strength being increased in the manner described above by the imposition of a self-bias voltage, an overhang portion is resputtered by process gas ions, and coverage of a hole""s inner surface can be improved. However, the ions also strike locations other than the overhang portion with high energy, and so cause resputtering of the thin film which is in the process of being built up or deposited. Consequently, the overall film deposition rate becomes slower and productivity falls.
There is also the problem that, as the result of the process gas ions being incident at high energy, process gas ions become admixed in the thin film, so causing a lowering of the quality of the thin film that is produced. Also, when the ion incidence quantity becomes large there is excessive accumulation of charge, and hence a problem of electrical damage to a device.
The present invention is one which has been devised in order to minimize or resolve such problems, and it has the technological significance that it provides a thin film fabrication method and a thin film fabrication apparatus with which, while the coverage of hole inner surfaces is improved, lowering of the film deposition rate is suppressed, and no harm is done to the quality of films which are produced or to manufactured device characteristics, etc.
In order to resolve the above problems, a structure is used in which, in a thin film fabrication method in which a plasma is formed in a space facing a surface of a substrate, the substrate surface is biased relative to the plasma space potential by imparting a set potential to the substrate surface, and the bias causes a set thin film to be produced on the substrate surface as ions in the plasma are caused to be incident on the substrate surface.
The biasing is preferably effected by imposing a voltage in pulse form on the substrate, the frequency of this voltage in pulse form is less than the oscillation frequency of the ions in the plasma, and the pulse period, pulse width and pulse height are controlled in a manner such that the quantity and energy of incidence of the ions on the substrate become optimum.
In the method described above, the waveform of the voltage in pulse form preferably contains a pulse for ion incidence and a pulse for relaxation whose polarity is different from that of the pulse for ion incidence.
In the method described above, the width of the pulse for relaxation is preferably shorter than a time which is the width of pulse for ion incidence deducted from the pulse period, and there is a time band in which neither a pulse for ion incidence nor a pulse for relaxation is imposed.
In the method described above, the pulse period, pulse width and pulse height are preferably controlled in a manner such that, in one pulse period, the incidence energy of the ions temporarily crosses a sputtering threshold value which is the lowest-limit energy value needed to effect sputtering of a thin film which is produced on the substrate surface.
In the method described above, the voltage in pulse form is preferably imposed indirectly on the substrate with the interposition of a dielectric.
In order to resolve the above problems, a structure is used in which, in a thin film fabrication apparatus which comprises a processing chamber whose interior is pumped out by a vacuum pump system, a substrate holder which holds a substrate in a set position in this processing chamber, a gas delivery system which introduces a set process gas into the processing chamber and a plasma generation means for producing a plasma in the processing chamber, and in which a set thin film is produced on the surface of a substrate held by the substrate holder. There is also provided a biasing device which causes ions in a plasma to be incident on the substrate surface by biasing the substrate surface relative to the plasma space potential by imparting a set potential to the substrate surface, and the biasing device preferably imposes a voltage in pulse form on the substrate, the frequency of this pulse being less than the oscillation frequency of the ions in the plasma. The structure further comprises a control section which controls the pulse period, pulse width and pulse height in a manner such that the quantity and energy of incidence of the ions on the substrate become optimum.
In the structure described above, the control section preferably effects control in a manner such that a voltage with a waveform which contains a pulse for ion incidence and a pulse for relaxation whose polarity is different from that of the pulse for ion incidence.
In the structure described above, the control section preferably effects control in a manner such that the width of the pulse for relaxation is shorter than a time which is the width of the pulse for ion incidence deducted from the pulse period, and there is a time band in which neither a pulse for ion incidence nor a pulse for relaxation is imposed.
In the structure described above, the control section preferably controls the pulse period, pulse width and pulse height in a manner such that, in one pulse period, the incidence energy of the ions temporarily crosses a sputtering threshold value which is the value of the lowest-limit necessary energy for effecting sputtering of a thin film which is produced on the substrate surface.
In the structure described above, a bias electrode is preferably installed facing the substrate with the interposition of a dielectric, and the biasing device imposes the voltage in pulse form on this bias electrode.